Fluid production wells having insufficient pressure are unable to flow liquids to the surface by natural means. Such wells require some form of energy or lift to transfer fluids to the surface.
Several artificial lift systems exist to extract the liquids from liquid-bearing reservoirs. In the case of lifting oil from wells, conventional lifting units include the beam pump and the surface hydraulic piston drive. Both of these lift units are situated at the surface of the well and lift fluid to the surface by “stroking” production tubing or rods inside production casing and/or well casing. The production tubing or rods is/are connected to a wellbore pump configuration, comprising a chamber and a check valve, which allows fluid to enter on the down-stroke and to be lifted to the surface on the up-stroke. These conventional lift units are supplied power from combustion engines or electric drives.
Beam pumps and surface hydraulic piston drives come in many sizes and are used extensively worldwide. U.S. Pat. Nos. 3,376,826; 3,051,237 and 4,296,678 are all examples of the use of a beam drive for a sucker string actuated pump. U.S. Pat. No. 4,403,919 is an example of a surface powered hydraulic pumping unit.
There are many drawbacks associated with the use of conventional beam pumps and surface hydraulic piston drives. These units are large, obtrusive and considered unsightly in many sensitive regions. Further, the tubing and/or rods from within the wellbore must extend outside the well through a stuffing box to connect the drive units to same. The stuffing box prevents the wellbore fluids from escaping to the surrounding surface environment, however, rarely is this completely successful, thereby resulting in hydrocarbon contamination of the ground surrounding the wellhead.
There are additional drawbacks to the use of conventional beam pumps and surface hydraulic piston drives. These units present a hazard to workers in the surrounding area as a result of exposure to surface moving parts. Further, beam pumps often experience alignment problems resulting in stress on the rods, undue wear and eventual failure. Finally, there are numerous dangers to personnel associated with assembly, transportation, installation, operation and maintenance due to the size of the units and their many moving parts.
U.S. Pat. No. 4,745,969 provides for a hydraulic/mechanical system for pumping oil wells that has a surface unit that can be hung inside of the well casing, so that there are no mechanical working parts outside of the well casing except for surface pipeline connections. However, this in-casing hydraulic jack system must be suspended from 20 to 40 feet below the surface of the ground, depending upon the required stroke. Further, the hydraulic jack unit is sealed within the well casing, resulting in a casing interior space for collecting reservoir fluid above the sealing means; this could result in leakage from the casing interior space to the environment, especially when lifting the hydraulic jack from the casing.
To address such issues, recently-allowed U.S. patent application Ser. No. 10/331,491 (Publication No. 2004/0112586), having one inventor in common with the present invention, teaches a wellhead hydraulic drive unit to operate various styles of downhole pumps, which is installed as an integral part of a wellhead thereby eliminating the need for a stuffing box. The wellhead hydraulic drive unit taught therein comprises a hollow hydraulic cylinder having a piston positioned therein, a hydraulic fluid supply means attached to the hydraulic cylinder for producing reciprocation of the piston within the hydraulic cylinder, a hollow ram means slideably received within the inner wall of the hydraulic cylinder and connected to the piston for reciprocation in response to the piston, and a production tube means inserted through the ram means for enabling well fluid to be discharged from the well.
While this unit did address many needs in the field of artificial lift systems, the present inventors perceived a need for increased adaptability and versatility, particularly noting the need to be able to install and remove an entire downhole pump assembly into a well without having to remove the installed wellhead hydraulic drive unit.